Catalyst and process for producing methacrylic acid

ABSTRACT

A catalyst composition useful for the oxidation of unsaturated aldehydes, particularly the oxidation of methacrolein to produce methacrylic acid, comprises the combination of oxides of molybdenum, copper, phosphorus, antimony, and cesium and/or calcium and optionally may include one or more of the elements Ni, Zn, Ru, Rh, Pd, Pt, As, K, Rb, Sr, Ba, Cr, V, Nb, W, Mn, Re, and rare earth metals including La.

CROSS-REFERENCE TO OTHER APPLICATIONS

This is a continuation-in-part of U.S. Ser. Nos. 973,354, 972,745,972,743 and 973,495, all filed Dec. 26, 1978, 027,632, 027,633, 027,634,and 027,635, all filed Apr. 6, 1979, and U.S. Ser. No. 047,860 filedJune 12, 1979.

PRIOR ART

This invention relates to a process and catalyst for the vapor-phaseoxidation with molecular oxygen of methacrolein to methacrylic acid.

It is well known that unsaturated acids, such as acrylic acid andmethacrylic acid, can be produced by the vapor-phase oxidation of thecorresponding unsaturated aldehydes by means of molecular oxygen in thepresence of a suitable oxidation catalyst. A variety of catalystcompositions have been proposed for this purpose. Many such compositionscomprise the oxides of molybdenum and phosphorus in association with theoxides of various other elements, both metallic and non-metallic. Ofinterest is U.S. Pat. No. 3,395,178 which discloses a preferredmolybdenum-phosphorus atomic ratio of 10:1 which is considered to beoptimum for a catalyst consisting only of those two elements anddisposed on a silicon carbide support. Also, British Pat. No. 1,498,595indicates that phosphorus is effective inmolybdenum-vanadium-phosphorus-cerium catalysts and that with a ratio ofMo:P greater than 12:3 the activity is decreased. However, in themolybdenum-phosphorus-vanadium-alkali metal catalyst of U.S. Pat. No.4,075,244, the phosphorus content was varied between 0.5 and 3 (relative to Mo₁₂) and no particular significance was noted.

British Pat. No. 1,430,337 and U.S. Pat. No. 4,000,088 propose the useof a catalyst composition in which the oxides of molybdenum andphosphorus are combined with the oxides of antimony, and copper andoptionally with chromium. The catalyst does not contain cesium orcalcium.

U.S. patents disclosing related catalysts which may contain cesium andcalcium include U.S. Pat. Nos. 4,051,179, 4,042,533, and 4,042,625. In'179 the basic constituents are molybdenum, phosphorus, and arsenic.Copper and vanadium are treated as alternatives and while an alkalimetal must be included, antimony and calcium are considered optional. In'533 molybdenum, tungsten and vanadium are required, while copper,phosphorus, and the alkali and alkaline earth elements are optional andantimony is lacking. In '625, magnesium and the alkali metals are addedto molybdenum-phosphorus catalysts, while calcium is optional and copperand antimony are lacking. Examples of the catalyst include phosphoruslevels of 1, 2, and 3 relative to Mo₁₂, but no particular significancewas noted.

In U.S. Pat. No. 4,045,478 the use of calcium in molybdenum-phosphoruscatalysts is taught. The catalyst lacks copper and the alkali metalswhile antimony is considered to be only an optional ingredient.

U.S. Pat. No. 3,976,688 teaches a molybdenum-phosphorus catalystcontaining an alkali metal and a group of optional elements whichinclude barium, but not calcium. Copper and antimony are lacking.

The catalyst disclosed in U.S. Pat. No. 4,042,533 also contains rheniumas an optional ingredient. Still another prior art patent disclosing theuse of rhenium as an optional ingredient is U.S. Pat. No. 3,956,378.While this catalyst requires the presence of molybdenum and antimony, itlacks the alkali and alkaline earth metals and copper and phosphorus areonly optional ingredients.

Many other elements have been used in catalysts for preparation ofmethacrylic acid by oxidation of methacrolein. U.S. patents whichdisclose the use of elements of interest with respect to the presentinvention include U.S. Pat. Nos. 4,001,316; 4,072,708; 4,138,365;3,557,199; and 3,985,680. In these patents, as well as those previouslymentioned, the catalysts differ from those to be disclosed hereinafter.The prior art catalysts generally are found to require elements notincluded in the catalysts of the present invention or some of theessential elements of the present catalysts are lacking in prior artcatalysts.

It has been found that catalysts for oxidation of methacrolein tomethacrylic acid have the characteristic property of remaining stablefor a period of time and then, without warning, of beginning a rapiddecline in activity. Consequently, an increase in the useful activity ofsuch catalysts has been sought.

Despite the many disclosures of the prior art, an improved catalyst ofthis type is not developed merely by randomly selecting a group of themany elements which have been disclosed. Small changes in compositionmay be very important in achieving improved catalyst performance andparticularly in optimizing the catalyst composition to suit, not only aspecific reaction, but the desired operating conditions also. The pointis well illustrated by the improved catalyst formulations to bedescribed hereinafter.

SUMMARY OF THE INVENTION

It has been discovered that when using the catalysts to be described toproduce methacrylic acid by vapor phase oxidation of methacrolein, it ispossible to achieve both high activity and high selectivity for extendedperiods of time. Broadly, the catalyst composition comprises oxides ofmolybdenum, copper, phosphorus, antimony, and cesium and/or calcium andthe composition may include one or more elements of Ni, Zn, Ru, Rh, Pd,Pt, As, K, Rb, Sr, Ba, Cr, V, Nb, W, Mn, Re, and rare earth metalsincluding La.

The catalyst composition used in the process of the invention also maybe expressed by the following general formula:

    Mo.sub.12 Cu.sub.a P.sub.b Sb.sub.c A.sub.d B.sub.e O.sub.x

wherein A is cesium and/or calcium and B is Ni, Zn, Ru, Rh, Pd Pt, As,K, Rb, Ca, Sr, Ba, Cr, V, Nb, W, Mn, Re, and rare earth metals includingLa, and where a-e and x indicate the atomic ratio of each component and,when a is 0.05-3, b is 0.1-5, c is 0.01-1, d is 0.1-3, e is 0-3,preferably 0.01-3, and x has a value which is determined by the valenceand proportions of the other elements in the catalyst. Preferably, bwill be 0.5-3, more preferably 1-2, and most preferably about 1.2-1.8.

Specific embodiments of catalysts according to the invention includecatalysts containing cesium or calcium, alone or with rhenium, accordingto the following formulas: Mo₁₂ Cu_(a) P_(b) Sb_(c) Cs_(d) O_(x) or Mo₁₂Cu_(a) P_(b) Sb_(c) Ca_(d) O_(x) ; where a-d and x have the range ofvalues given above with the exception that b=1-2, preferably 1.2-1.8,and most preferably b=1.3=1.7; and Mo₁₂ Cu_(a) P_(b) Sb_(c) Cs_(d)Re_(e) O_(x) or Mo₁₂ Cu_(a) P_(b) Sb_(c) Ca_(d) Re_(e) O_(x) where a-eand x have the range of values given above with the exception thatb=1-2, preferably 1.2-1.8, and most preferably b=1.3-1.7. Still anothercatalyst of the invention is represented by the formula Mo₁₂ Cu_(a)P_(b) Sb_(e) Ca_(d) W_(e) O_(x), again where a-e and x have the valuesgiven above with the exception that b=1-2, preferably b=1.2-1.8.

When such a catalyst as has been described is in contact with avapor-phase mixture of methacrolein, molecular oxygen, steam, andnitrogen at typical temperatures in the range of 250°-400° C. andpressures in the range of 0-5 atmospheres, excellent activity andselectivity to the production of methacrylic acid is obtained forextended periods of time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Catalyst Composition andPreparation

The catalyst of the invention comprises oxides or oxygen-containingcompounds of molybdenum, copper, phosphorus, antimony, and cesium and/orcalcium and optionally may include members of a group of elements to benamed. The catalyst may be represented by the general formula:

    Mo.sub.12 Cu.sub.a P.sub.b Sb.sub.c A.sub.d B.sub.e O.sub.x

wherein A is cesium and/or calcium and B is Ni, Zn, Ru, Rh, Pd, Pt, As,K, Rb, Sr, Ba, Cr, V, Nb, W, Mn, Re and rare earth metals including La,and where a-e and x indicate the atomic ratio of each component relativeto Mo₁₂ and, when a is 0.05-3, b is 0.1-5, c is 0.01-1, d is 0.1-3, e is0-3, preferably 0.01-3, x is a value determined by the valence andproportions of the other elements in the catalyst. Preferably, b will be0.5-3, more preferably 1-2, and most preferably about 1.2-1.8. Preferredcatalysts include those in which component B is tungsten or rhenium.Other elements, which may be included in minor amounts in the catalystformulation in order to promote catalyst activity or selectivity andwithout losing the advantages to be shown for the general formula, areconsidered to be within the scope of the invention. The catalystcomposition may be regarded either as a mixture of oxides of the namedelements or as oxygen-containing compounds of the elements or both. Asprepared and/or under reaction conditions, the catalyst may containeither or both forms and both are intended to be included within thephrase "mixtures of oxides."

The catalyst composition is preferably used in unsupported form, e.g. inthe form of pellets or other like compressed shapes of various sizes,although conventional supports could be employed instead. Thecomposition may be formed in conventional manner using techniques wellknown to persons skilled in the art. For example, compounds ofmolybdenum, copper, phosphorus, antimony, cesium, and rhenium aredissolved in a small amount of water or other solvent, and the solutionsare then combined and evaporated to dryness, e.g. in a rotary dryer. Theseveral components can be introduced into solution in the form ofvarious salts or other compounds of convenient types and no specificform for the catalyst precursors is necessary. The use of ammoniumsalts, halides e.g. chlorides, nitrates or acid forms of the elements,e.g. phosphoric acid, are, however, particularly suitable. Preferably,however, aqueous solutions are employed and water-soluble forms of theelements are used. In some cases the solutions may have acids and/orbases added to them to facilitate dissolution of the catalystprecursors. For example, acids such as hydrochloric or nitric acid, orbases such as ammonium hydroxide, can be used as desired. The resultingpowder from the evaporation is then thoroughly dried and preferablyscreened to eliminate large particles which make it difficult to produceuniform compressed shapes, such as pellets. Typically, the powder ispassed through a 20-mesh screen. The powder is then mixed with anorganic binder which can be of any conventional type, such as polyvinylalcohol, and the mixture is thoroughly dried and again screened,typically to provide a 20-60 mesh size. The dried mixture is thenpreferably combined with a lubricant, again of any conventional type,such as stearic acid or graphite, and compressed into the desired shape,e.g. pelletized, the compressed shapes typically having heights anddiameters of 1/16 inch to 3/8 inch. Finally, the thus produced catalystcomposition is activated at high temperature for a prolonged period inaccordance with conventional practice in this art. For example, thepellets are placed in an oven or kiln, or in a tube through which air ispassed, at an elevated temperature (e.g. 300°-500° C., preferably325°-450° C.) for at least ten hours. In a particularly preferredactivation step, the temperature is raised at the rate of 20° C. perhour to a maximum of 420° C., preferably 320°-400° C., and thistemperature is maintained for 8 hours.

It will be understood that the foregoing description regardingpreparation of the catalyst in a form suitable for use in a vapor-phaseoxidation reaction is merely illustrative of many possible preparativemethods, although it is a particularly suitable method and is preferred.

Methods of Operation

The catalysts described are generally useful for the production ofunsaturated acids by oxidation with molecular oxygen of unsaturatedaldehydes, although the reaction of methacrolein to form methacrylicacid is of particular interest. Other possible starting materials arethe monoethylenically unsaturated aliphatic monoaldehydes of from 3 to 6carbon atoms, such as acrolein, crotonaldehyde, 2-methyl-2-butenal, andthe like, or mixtures thereof.

The reaction in which the catalyst compositions of this invention are ofparticular utility and in which they provide high conversions andselectivities involves contacting the catalyst with methacrolein andoxygen in the vapor phase, preferably also in the presence of steam anddiluents. When the catalyst of this invention is used in the vapor-phaseoxidation of methacrolein to form methacrylic acid, the oxidationconditions employed are those generally associated with this reaction,although it is preferred that the molar ratio of oxygen to methacroleinshould be kept at a high value near the flammable range. Once reactionis begun, it is self-sustaining because of its exothermic nature. Avariety of reactor types may be employed such as fluid or fixed bedtypes, but reactors having the catalyst disposed inside a multiplicityof heat exchanger tubes are particularly useful and convenient.

The gaseous feed to the reactor contains appropriate concentrations ofmethacrolein, oxygen and steam and usually an inert gas is also present,such as nitrogen and the like. The oxygen is usually added as such or asair, which may be enriched with oxygen. As mentioned, conventionaloxidation conditions can be employed but it is a feature of the catalystof this invention that methacrolein can be present in concentrations ofmore than 5 up to about 20 volume percent of the total feed with apreferred range of more than 5 up to about 15 volume percent. In generalat least 6 volume percent of the aldehyde is used in the feed. Thecorresponding ranges for oxygen are 3 to 15 volume percent, preferably 5to 12 volume percent and for steam up to 50 volume percent, preferably 5to 35 volume percent, the balance being the inert gas or gases.

The temperature of the reaction should, for best results, be within therange of from about 270° to 450° C., preferably 280°-400° C., and theoptimum temperature range is 290° to 325° C. Because the reaction isexothermic, means for conducting the heat away from the reactor arenormally employed to avoid a temperature increase which favors thedestruction of methacrolein by complete oxidation to carbon oxides andwater. The reactor temperature may be controlled by conventional methodssuch as by surrounding the catalyst-containing tubes with a molten saltbath.

The reaction may be conducted at atmospheric, superatmospheric orsubatmospheric pressure. Preferably, however, pressures are employedranging from atmospheric up to about 8 kg/cm² absolute, preferably up toabout 6.3 kg/cm² absolute, and most preferably up to about 4.5 kg/cm²absolute.

The unsaturated acid product may be recovered by a number of methodswell known to those skilled in the art. For example, the acid may becondensed, or scrubbed with water or other suitable solvents, followedby separation of the unsaturated acid product from the scrubbing liquid.The gases remaining after the acid-removal step may be recycled to thereaction preferably after removal of CO₂ by conventional means, e.g.,absorption in aqueous carbonate solution.

The features of the invention will be more readily apparent from thefollowing specific examples of typical catalyst preparation and its usein the oxidation of methacrolein. It will be understood, however, thatthese examples are for the purpose of illustration only and are not tobe interpreted as limiting the invention.

EXAMPLE 1 Catalyst Preparation

In 750 cc of water are dissolved 636 grams of (NH₄)₆ Mo₇ O₂₄.4H₂ O. Then21.7 grams of Cu(NO₃)₂.3H₂ O are dissolved in 100 cc of water, 58.4grams of CsNO₃ are dissolved in 150 cc of water, 20.5 grams of SbCl₃ aredissolved in a mixture of 30 cc of water, and 10 cc of concentrated HCland 34.5 grams of H₃ PO₄ are dissolved in a mixture of 100 cc of waterand 50 cc of 58% NH₄ OH solution. These solutions are mixed with 400 ccof 58% NH₄ OH and fed to a rotary dryer of 4000 cc capacity and themixture is evaporated to dryness at a temperature reaching a maximum of140°-200° C. The resulting powder is removed from the dryer and dried inan oven at 200° C. for 4 hours. The dried powder is screened through a20-mesh screen, a 4% aqueous solution of polyvinyl alcohol is added insufficient quantity to make a damp mixture and this mixture is dried at75°-80° C. until the moisture content falls to 2-4 wt.%. The driedmixture is then screened to 20-60 mesh size particles, and about 2-6% ofstearic acid powder is thoroughly mixed with it. The resulting mixtureis then pelletized to form pellets of 3/16 inch height and diameter inwhich the catalyst components molybdenum, copper, phosphorus, antimony,cesium are present (by calculation) in the atomic ratios of 12, 0.3, 1,0.3 and 1, respectively. The pellets are then activated in an oven byheating them to 100° C. in one hour and then raising the temperaturegradually at a rate of about 20° C. per hour to 370° C. and maintainingthem at this temperature for 8 hours. The catalyst is tested accordingto the procedure of Example 2.

EXAMPLE 2 Catalyst Testing

A 150 cc quantity of the catalyst composition of Example 1 is placed ina reactor defined by a 1/2"×90" stainless steel pipe, the reactor pipebeing filled with 50 cc of inert filler (silicon carbide) below thecatalyst bed and 100 cc of the inert filled above the catalyst bed inconventional manner to insure uniform temperature contact with thecatalyst. Nitrogen-diluted mixtures containing methacrolein, oxygen andsteam are fed to the reactor at a pressure of 1.74 kg/cm² (absolute) andat a space velocity of about 1200 hr⁻¹. The term "space velocity" isused in its conventional sense to mean liters of gas (at standardtemperature and pressure) per liter of catalyst per hour. The feedcomposition is approximately, by volume, 6-7% methacrolein, 11-12%oxygen and 20% steam, the balance being nitrogen, determination beingmade on a wet basis. The reaction is run continuously and the exit gasis analyzed at intervals of several hours. Analyses are carried out bymeans of gas chromatography and by infrared spectrography usingconventional techniques. The average amount of methacrylic acid producedis determined periodically and the reactor temperature is adjusted asnecessary to obtain the desired yield, that is, the product of theconversion and the selectivity, which for purposes of the comparisons tobe made is about 0.15 gm of methacrylic acid per hour per gram ofcatalyst.

EXAMPLE 3

A catalyst is prepared according to the method of Example 1 except thatthe phosphorus content is increased to provide a catalyst having thefollowing nominal composition (by calculation):

    Mo.sub.12 Cu.sub.0.3 P.sub.1.5 Sb.sub.0.3 Cs.sub.1 O.sub.x

The catalyst is tested according to the method of Example 2.

EXAMPLE 4

A catalyst is prepared according to the method of Example 1 but having ahigher phosphorus content than the catalyst of Example 3, providing acatalyst having the following nominal composition (by calculation):

    Mo.sub.12 Cu.sub.0.3 P.sub.2 Sb.sub.0.3 Cs.sub.1 O.sub.x

The catalyst is tested according to the method of Example 2.

The results of the tests carried out on the catalysts of Examples 1, 3,and 4 are summarized in the following Table I.

                  TABLE I                                                         ______________________________________                                                                       Selectivity to                                      Phosphorus         Temp.  Methacrylic                                                                            Activity                              Cat. Content.sup.a                                                                           Hours    °C.                                                                           Acid.sup.b                                                                             Coeff. K.sup.c                        ______________________________________                                        1    1          35      293    77       23                                                   80       323    74.5     6                                     3    1.5       150      298    77.6     26                                                   200      296    79.7     26                                    4    2         26       320    70       10                                                   122      324    65       7                                     ______________________________________                                         .sup.a relative to Mo.sub.12                                                  .sup.b percent of methacrolein reacted which is converted to methacrylic      acid                                                                          .sup.c a value calculated from experimental data to provide a measure of      catalyst activity and derived from the                                        K = F . X . S . C.sup.E/RT where:                                             F = methacrolein concentration in                                             S = space velocity of feed gas                                                X = conversion of methacrolein                                                E = activation energy, 25,000 k cal/mol.                                      R = gas constant                                                              T = absolute temperature                                                 

As explained in Example 2 the catalysts are operated to provide aconstant predetermined yield of methacrylic acid. In order to provide aproper comparison, the performance of a catalyst is reported at theperiod of time where the catalyst activity has stabilized after aninitial break-in period. This may mean, as in Table I, that the timeselected for comparisons is not the same if the catalyst performancediffers significantly. A stable period at 150-200 hours could beestablished readily for the catalyst of Example 3, while the catalystsof Examples 1 and 4 had relatively poorer performance and theiractivities declined earlier, as indicated by the data. It is clear thatthe catalyst of Example 3 is superior to those of Examples 1 and 4,since it operated consistently at a lower temperature and with higheractivity and selectivity after the other catalysts had lost significantactivity. Thus, is is concluded that catalysts containing molybdenum,copper, antimony, and cesium are sensitive to the phosphorus content. Anoptimum level should be found between a phosphorus level of 1 and 2(relative to Mo₁₂). It is believed that the optimum level is locatedbetween P₁.2 and P₁.8, particularly between P₁.3 and P₁.7.

EXAMPLE 5

A catalyst corresponding to that of Example 1 is prepared by the samegeneral technique except that 5 grams of perrhenic acid dissolved in 100cc of water are included in the initial solution to provide rhenium in acatalyst having the following nominal composition (by calculation):

    Mo.sub.12 Cu.sub.0.3 P.sub.1 Sb.sub.0.3 Cs.sub.1 Re.sub.0.07 O.sub.x

EXAMPLE 6

A catalyst is prepared according to the general method of Example 5except that the amount of phosphorus is increased to produce a catalysthaving the following nominal composition (by calculation).

    Mo.sub.12 Cu.sub.0.3 P.sub.1.5 Sb.sub.0.3 Cs.sub.1 Re.sub.0.07 O.sub.x

The catalyst is tested according to the methods of Example 2.

EXAMPLE 7

A catalyst is prepared according to the method of Example 5 but theamount of phosphorus is doubled to provide a catalyst having thefollowing nominal composition (by calculation):

    Mo.sub.12 Cu.sub.0.3 P.sub.2 Sb.sub.0.3 Cs.sub.1 Re.sub.0.07 O.sub.x

The catalyst is tested according to the procedures of Example 2.

The results of testing the catalysts of Examples 5, 6, and 7 aresummarized in the following Table II, to which the footnotes of Table Ialso apply.

                  TABLE II                                                        ______________________________________                                                                      Selectivity to                                                                          Activity                                    Phosphorus        Temp. Methacrylic                                                                             Coeff. K                              Cat.  Content(a)                                                                              Hours   °C.                                                                          Acid(b)   (c)                                   ______________________________________                                        5     1         150     292   75        26                                                    200     293   75.5      26                                    6     1.5       150     279   77        46.6                                                  250     279   77        40                                    7     2         150     311   73.6      11.6                                                  250     311   76        12                                    ______________________________________                                    

As in Table I, the catalysts are operated to provide the same yield ofmethacrylic acid and the performance is reported during a stable periodof operation after the initial break-in of the catalyst. It will beclear that the catalysts having rhenium added have better performancethan those of Table I, which contain no rhenium, since they have higheractivity and suggest improved aging characteristics, especially withrespect to the comparison of catalysts 5 and 7 with catalysts 1 and 4.The rhenium-containing catalysts also are sensitive to the phosphoruscontent and an optimum value again appears between P₁ and P₂ (relativeto Mo₁₂). It is believed that the optimum value is between P₁.2 andP₁.8, particularly between P₁.3 and P₁.7.

Taking the results of Tables I and II together, it may be concluded thatthe ratio of molybdenum to phosphorus for catalysts of this type willshow an optimum performance at ratios between 12/1 to 12/2.

EXAMPLE 8

A catalyst is prepared according to the general method of Example 1except that 79.2 gms of Ca(C₂ H₃ O₂)₂.X H₂ O are substituted for cesiumnitrate and no aqueous ammonia is added and a catalyst having thefollowing nominal composition (by calculation) is produced:

    Mo.sub.12 Cu.sub.0.3 P.sub.1 Sb.sub.0.3 Ca.sub.1.5 O.sub.x

This catalyst is tested according to the methods of Example 2 and thesignificance of the phosphorus to molybdenum ratio to catalystperformance is shown. The catalyst is operated at about 304° C. for aperiod between about 20 to 80 hours with a selectivity of 76.3 and anactivity coefficient of 18.6.

EXAMPLE 9

A catalyst is prepared according to Example 8 except that 5 grams of Re₂O₇ dissolved in 100 cc of water is included in the solution to providerhenium a catalyst having the following nominal composition (bycalculation):

    Mo.sub.12 Cu.sub.0.3 P.sub.1 Sb.sub.0.3 Ca.sub.1.5 Re.sub.0.07 O.sub.x

The catalyst is tested according to the method of Example 2. Thesignificance of the phosphorus to molybdenum ratio to catalystperformance is shown. The catalyst is operated at about 286° C. for aperiod between about 30 to 200 hours with a selectivity of 75.2 and anactivity coefficient of 42.

EXAMPLE 10

A catalyst is prepared according to the general method of Example 9except that instead of rhenium, tungsten is included and no aqueousammonia is used to provide a catalyst having the following nominalcomposition (by calculation):

    Mo.sub.12 Cu.sub.0.3 P.sub.1 Sb.sub.0.3 Ca.sub.1 W.sub.0.5 O.sub.x

The catalyst is tested according to the method of Example 2 and thesignificance of the phosphorus to molybdenum ratio to catalystperformance is shown. For a period between about 20 to 170 hours thecatalyst is operated at about 302° C., with a selectivity of about 76.5and an activity coefficient of about 22.4.

EXAMPLE 11

A catalyst is prepared according to the general method of Example 1except that the phosphorus level is increased and palladium is includedto produce a catalyst having the following nominal composition:

    Mo.sub.12 Cu.sub.0.3 P.sub.2 Sb.sub.0.3 Cs.sub.1 Pd.sub.0.03 O.sub.x

The catalyst is tested under the conditions of Example 2 and thesignificance of the phosphorus to molybdenum ratio with respect tocatalyst performance is shown.

EXAMPLE 12

A catalyst is prepared according to the general method of Example 1except that the phosphorus level is increased and rubidium is added toproduce a catalyst having the following nominal composition:

    Mo.sub.12 Cu.sub.0.3 P.sub.1.75 Sb.sub.0.3 Cs.sub.0.5 Rb.sub.0.5 O.sub.x

The catalyst is tested under the conditions of Example 2 and thesignificance of the phosphorus to molybdenum ratio with respect tocatalyst performance is shown.

EXAMPLE 13

Other suitable catalysts in accordance with the invention are preparedaccording to the general method of Example 1 and have the followingnominal compositions:

Mo₁₂ Cu₀.3 P₂ Sb₀.3 Cs₀.3 Ca₀.7 Cr₀.3 O_(x)

Mo₁₂ Cu₀.3 P₂ Sb₀.3 Cs₀.3 K₀.7 V₀.3 O_(x)

Mo₁₂ Cu₀.3 P₂ Sb₀.3 Cs₀.3 Sr₀.7 Nb₀.3 O_(x)

Mo₁₂ Cu₀.3 P₁.7 Sb₀.3 Cs₀.3 Ba₀.7 Mn₀.3 O_(x)

Mo₁₂ Cu₀.3 P₁.2 Sb₀.3 Cs₁ La₀.7 Ni₀.3 O_(x)

Mo₁₂ Cu₀.3 P₁ Sb₀.3 Cs₀.3 Ce₀.7 Zn₀.3 O_(x)

Mo₁₂ Cu₀.3 P₁ Sb₀.3 Cs₁ Ru₀.1 O_(x)

Mo₁₂ Cu₀.3 P₀.5 Sb₀.3 Cs₁ Rh₀.03 O_(x)

Mo₁₂ Cu₀.3 P₁ Sb₀.3 Cs₀.3 Pt₀.03 O_(x)

Mo₁₂ Ca₀.3 P₁.5 Sb₀.3 Cs₁ W₀.5

The catalysts are tested under the conditions of Example 2 and thesignificance of the phosphorus to molybdenum ratio with respect tocatalyst performance is shown.

What is claimed is:
 1. A catalyst suitable for the vapor-phase oxidationof methacrolein to methacrylic acid consisting essentially of thecomposition expressed by the formula:

    Mo.sub.12 Cu.sub.a P.sub.b Sb.sub.c A.sub.d B.sub.e O.sub.x

where: a=0.05-3; b=0.1-5; c=0.01-3; d=0.1-3; e=0.01-3; and x=a valuedetermined by the valence and proportions of the other elements of theformula, where A is Cs and/or Ca and B is one more elements selectedfrom the group consisting Ni, Zn, Ru, Rh, Pd, Pt, As, K, Rb, Sr, Ba, Cr,V, Nb, W, Mn, Re, and rare earth metals including La.
 2. A catalyst ofclaim 1 wherein b=1-2.
 3. A catalyst of claim 2 wherein b=1.2-1.8.
 4. Acatalyst of claim 1 wherein A is Cs.
 5. A catalyst of claim 4 whereinb=1.2-1.8.
 6. A catalyst of claim 5 wherein b=1.3-1.7.
 7. A catalyst ofclaim 1 where A is Ca.
 8. A catalyst of claim 7 wherein b=1-2.
 9. Acatalyst of claim 8 wherein b=1.2-1.8.
 10. A catalyst suitable for thevapor-phase oxidation of methacrolein to methacrylic acid consistingessentially of the composition expressed by the formula:

    Mo.sub.12 Cu.sub.a P.sub.b Sb.sub.c Cs.sub.d O.sub.x

where: a=0.05-3; b=1-2; c=0.01-3; d=0.1-3; and x=a value determined bythe valence and proportions of the other elements of the formula.
 11. Acatalyst of claim 10 wherein b=1.2-1.8.
 12. A catalyst of claim 11wherein b=1.3-1.7.
 13. A catalyst suitable for the vapor-phase oxidationof methacrolein to methacrylic acid consisting essentially of thecomposition expressed y the formula:

    Mo.sub.12 Cu.sub.a P.sub.b Sb.sub.c Ca.sub.d O.sub.x

where: a=0.05-3; b=1-2; c=0.01-3; d=0.1-3; and x=a value determined bythe valence and proportions of the other elements of the formula.
 14. Acatalyst suitable for the vapor-phase oxidation of methacrolein tomethacrylic acid consisting essentially of the composition expressed bythe formula:

    Mo.sub.12 Cu.sub.a P.sub.b Sb.sub.c Cs.sub.d Re.sub.e O.sub.x

where: a=0.05-3; b=1-2; c=0.01-3; d=0.1-3; e=0.005-0.5; and x=a valuedetermined by the valence and proportions of the other elements of theformula.
 15. A catalyst of claim 14 wherein b=1.2-1.8.
 16. A catalyst ofclaim 15 wherein b=1.3-1.7.
 17. A catalyst suitable for the vapor-phaseoxidation of methacrolein to methacrylic acid consisting essentially ofthe composition expressed by the formula:

    Mo.sub.12 Cu.sub.a P.sub.b Sb.sub.c Ca.sub.d Re.sub.e O.sub.x

where: a=0.05-3; b=1-2; c=0.01-3; d=0.1-3; e=0.005-0.5; and x=a valuedetermined by the valence and proportions of the other elements of theformula.
 18. A catalyst suitable for the vapor-phase oxidation ofmethacrolein to methacrylic acid consisting essentially of thecomposition expressed by the formula:

    Mo.sub.12 Cu.sub.a P.sub.b Sb.sub.c Ca.sub.d W.sub.e O.sub.x

where: a=0.05-3; b=1-2; c=0.01-3; d=0.1-3; e=0.01-3; and x=a valuedetermined by the valence and proportions of the other elements of theformula.
 19. The catalyst of claim 18 further comprising arsenic.